A conventional ultrasound imaging system comprises an array of ultrasonic transducer elements which transmit an ultrasound beam and receive the reflected beam from the object being studied. Specifically, a series of measurements is performed in which a focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next. The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each receiving transducer element.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. Resolution of a scan line is a result of the directivity of the associated transmit and receive beam pair.
The output signals of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
In medical ultrasound imaging systems of the type described hereinabove, it is desirable to optimize the SNR. Additional SNR can be used to obtain increased penetration at a given imaging frequency or to improve resolution by facilitating ultrasonic imaging at a higher frequency.
The use of Golay codes in ultrasound is well known in the area of non-destructive evaluation (NDE) using single-element fixed-focus transducers to inspect inanimate objects. Golay codes are also known in the medical ultrasound imaging community. However, Golay codes have not commonly been used in ultrasound imaging systems because dynamic focusing, tissue motion (effects not present in NDE) and nonlinear propagation effects were thought to cause unacceptable code degradation with corresponding range degradation.
U.S. patent application Ser. No. 09/063,109, filed Apr. 20, 1998 and assigned to the instant assignee, discloses a method and an apparatus for improving the SNR in medical ultrasound imaging by using Golay-encoded excitation of the transducer array. The SNR is improved by transmitting a pair of Golay-encoded base sequences consecutively on each beam at the same focal position and then decoding the beamsummed data. A pair of Golay-encoded base sequences are formed by convolving a base sequence with a Golay code pair after oversampling. A Golay code pair is a pair of binary (+1, -1) sequences with the property that the sum of the autocorrelations of the two sequences is a Kronecker delta function. An oversampled Golay sequence is the Golay sequence with zeroes between each +1 and -1, the number of zeroes being greater than or equal to one less than the length of the base sequence. The aforementioned property of Golay code pairs translates into two important advantages over codes in general: (1) Golay codes have no range sidelobes, and (2) Golay codes can be transmitted using only a bipolar pulser versus a more expensive digital-to-analog converter.
In practice, images produced using coded excitation can be degraded by code distortion. Bipolar codes degrade when the signal representing the positive code symbol no longer cancels out with the signal representing the negative code symbol after nonlinear propagation. There is need for a method to reduce code degradation due to nonlinear propagation and to implement coded excitation on systems which have only unipolar pulsers.